I still think the only way this would work (aside from having the hot tub in the green house, would be to capture lost heat. Now my hot tub is very well insulated, especialy above the tub. My hot tub is a self contained tub, the heater is contained in the tub enclosure. But I think this method would work even better with an inground hot tub. Idealy, if the hot tub is not installed yet, you could build an inground hot tub (the heater is seperate with an inground hot tub). Install the just the heater inside the greenhouse. Otherwise, the place I have found the most Waste heat with my Tub, is above the heater. So hputting some sort of heat exchanger above the heater to capture the heat lost from the heater would work very well.

I have finished major part of the project, still need to fix small things like paint work on the wooden boarder, arbor over waterfall, rails, aquatic plants and fish. here are some pics; Mia, thanks again for the pump and filters, it is working great, lift lot of water. Thank you all for all those tips to make it happen. Here is a link that might be useful: Hot Bath Tub Water Garden

jake, you hit the nail on the head. the pipes are colling off under the slab, the water flow thru the shower head is not enough to overcome this. due to the layout of the house, the pipes run a minimum of 120 ft under the concrete, and that is if they take the most direct route from heater to tub. if they go to the other bath first, and i know they do since it has almost instant hot water, it is probably closer to 150 ft of travel in 3/4" copper lines. i know the lines are not insulated under the slab, that is not required here even though should be. but then again with no building codes, nothing is required! i did not get around to replacing the shower head this weekend, but i did run an experiment. i turned on the shower as usual, and sure enough after 2 minutes of being hot, the water went luke warm at best. i then turned the diverter slightly so that maybe a half gallon a minute flow was going into the tub. within second the water was HOT! my wife said that was the first shower this winter that she got to shave her legs in hot water. time for a new shower head, hopefully one that will provide enough flow to stop teh water cooling in the pipes.

the problem is.. they are usually empty ... if you could actually find one full .. and test out the motors and what not ... then it might be worth it ... but i have to tell you.. moving one of these larger units is not easy nor fun ... and it would be a lot of backbreaking labor.. to find out it leaks.. or the circuitry is fried ... i didnt see where you are ... but running it.. in the great frozen north.. all winter long will be a hundred or two dollars per month ... and that is why i eventually got rid of mine ... i could not steep myself in there.. and relax ..... knowing it was costing me so much dough ... lol ... even in summer.. it wasnt cheap.. keeping it at temp ... as it takes hours for it to go up a degree or two ... so its not like you can knock the temp back.. like you might your furnace ... at night ... and think it will come back up to heat.. in an hour or two ... ken